Downhole isolation tools located and set to isolate a zone of well casing in the wellbore during stimulation or other service work include bridge plugs and frac plugs. Shown in FIG. 1, the isolation tool 1 includes an internal elongated mandrel 2 and a circular array of slips 4 mounted on the mandrel 2 at each end of the tool 1. Each slip 4 has an outer surface configured to engage an internal surface of the well casing. Each array of slips 4 is disposed next to a respective conical ring 6 mounted on the mandrel 2 for sliding under the inclined inner surfaces of the slips 4 in the array. In the middle of the isolation tool 1 is an elastomeric seal 8 between the conical rings 6. When a setting tool (not shown) pulls the mandrel 2 in the longitudinal direction, the seal 8 expands outward in the radial direction to seal the well casing. In addition, the conical rings 6 slide under the slips 4 and force the slips 4 outward in the radial direction into engagement with the well casing. The slips 4 lock the isolation tool 1 in place inside the well casing in such a way that the seal 8 remains compressed for sealing the well casing when the setting tool is removed.
Oftentimes it is most economical to manufacture the isolation tool to become permanently set in the well casing so that it must be drilled out destructively to unseal the well casing. Therefore, a number of downhole tool makers have replaced cast iron components of the isolation tool with composite components. Composite components can be drilled out faster than cast iron, and the drilled-out chips of composite material are lighter than cast iron so that the composite chips are more easily flushed out of the tubular member with drilling fluid. Historically, composite mandrels for bridge and frac plugs have been fabricated using manufacturing methods that are designed to form one-piece tubes, such as roll wrapping, filament winding, and pultrusion. However, compression molding a tube has drawbacks in that undesired flow lines are created causing low strength areas. What is needed then is a method of manufacturing a composite mandrel for bridge and frac plugs that alleviates problems of the prior art.